The present invention relates to an operational amplifier, and more particularly, it relates to a technique to stabilize the characteristics of an operational amplifier over a wide range of a supply voltage.
FIG. 6 illustrates an exemplified circuit configuration of a conventional operational amplifier. The operational amplifier of FIG. 6 includes a differential circuit 10, a level shift circuit 20, an output circuit 30 and a bias current source circuit 40.
In the differential circuit 10, a differential input transistor 11 changes differential currents Idf1 and Idf2 in accordance with a voltage difference between input signals SI1 and SI2. At this point, the sum of the differential currents Idf1 and Idf2 always accords with an operating current Idiff flowing through a differential current source transistor 13. Furthermore, an active load transistor 12 changes an output signal Sa of the differential circuit 10 in accordance with a difference between the differential currents Idf1 and Idf2.
The level shift circuit 20 shifts the output signal Sa of the differential circuit 10 by a voltage corresponding to the gate voltage V1 of a level shift transistor 21, so as to output the resultant signal as a signal Sb. This level shift voltage V1 depends upon an operating current Ils flowing through a level shift current source transistor 22.
The output circuit 30 has a push-pull configuration including a p-type first output stage transistor 31 and an n-type second output stage transistor 32. The first output stage transistor 31 receives, at its gate, the output signal Sa of the differential circuit 10, and the second output stage transistor 32 receives, at its gate, the output signal Sb of the level shift circuit 20. The signals Sa and Sb are respectively amplified by the first and second output stage transistors 31 and 32, resulting in outputting a signal SO as an output signal of the operational amplifier.
The bias current source circuit 40 supplies a voltage Vg, which is obtained by dividing the supply voltage with resistors 41 and 42, as the gate voltage of the differential current source transistor 13 and the level shift current source transistor 22.
In the case where a high frequency pulse signal, such as a PWM modulated audio signal, is used as an input signal to the operational amplifier, the operational amplifier is required to have a low distortion rate and a high through rate. According to examination made by the present inventor, however, it has been found that the conventional operational amplifier having the configuration as is shown in FIG. 6 cannot always exhibit good characteristics when the supply voltage is changed in a wide range, for example, between 3 V and 5 V.
First, the distortion characteristic of an operational amplifier will be examined.
FIG. 7 is a graph for showing the Ids-Vds characteristic of a transistor. In FIG. 7, Vds indicates a drain-source voltage, Ids indicates a drain current, Vgs indicates a gate-source voltage (gate voltage), and Vt indicates a threshold voltage. As is shown in FIG. 7, as the gate voltage Vgs of the transistor is increased, the range of the drain-source voltage Vds included in an unsaturation region is increased (Va&lt;Vb&lt;Vc), and the range of the drain-source voltage Vds included in a saturation region where the Vds-Ids characteristic linearly changes is decreased. Accordingly, when the gate voltages Vgp and Vgn of the output stage transistors 31 and 32 are increased, the ranges of their drain-source voltages Vds included in the saturation region are narrowed. As a result, a range of a voltage where the signal SO can be output without distortion is accordingly narrowed. In other words, as the gate voltages Vgp and Vgn of the output stage transistors 31 and 32 are higher, the output signal SO can be more easily distorted.
Therefore, in view of the distortion characteristic of an operational amplifier, it is desired that, when the supply voltage is changed, the level shift voltage V1 is similarly changed so as not to largely change the gate voltages Vgp and Vgn of the output stage transistors 31 and 32. Since the level shift voltage V1 depends upon the operating current Ils of the level shift circuit 20, in order to retain a good distortion characteristic over a wide range of the supply voltage, it is necessary to largely change the operating current Ils of the level shift circuit 20 in accordance with the change of the supply voltage.
For example, in order to set the gate voltages Vgp and Vgn of the output stage transistors 31 and 32 at 1 V, when the supply voltage is 5 V, the level shift voltage V1 is required to be set at 3 (=5-1-1) V and when the supply voltage is 3 V, the level shift voltage V1 is required to be set at 1 (=3-1-1) V. On the other hand, the following relationship holds: EQU Ids=K(Vgs-Vt).sup.2
wherein K is a constant depending upon the size of a transistor. Therefore, in order to change the level shift voltage V1 from 1 V to 3 V, it is necessary to change the operating current Ils of the level shift circuit 20 from K(3-Vt).sup.2 to K(1-Vt).sup.2. When the threshold voltage Vt is assumed to be 0.8 V, it is necessary to change the operating current Ils of the level shift circuit 20 in a wide range from 4.84 K to 0.04 K.
Next, the through rate characteristic of an operational amplifier will be examined.
As a result of experiments made by the present inventor, it has been found that the through rate of an operational amplifier is higher as the operating current Idiff of the differential circuit 10 is larger. In particular, when a signal with a small pulse width is used as an input signal, the operating current Idiff is preferably sufficiently increased so that the edge of an output signal can be steep enough for the pulse not to collapse. On the other hand, when the operating current Idiff is large, the gate voltages of the transistors included in the differential circuit 10 are accordingly increased. Therefore, if the operating circuit Idiff is excessively increased, there is a possibility that the differential circuit 10 cannot be normally operated when the supply voltage is low.
Accordingly, in view of the through rate characteristic, it is necessary to set the operating current Idiff so as to obtain a desired through rate within a range where the differential circuit 10 can be normally operated. For example, the operating current Idiff is preferably controlled to be substantially constant regardless of the supply voltage.
However, in the conventional operational amplifier of FIG. 6, the voltage Vg generated by the bias current source circuit 40 is supplied to both the differential current source transistor 13 and the level shift current source transistor 22 as their gate voltages, and the operating current Idiff of the differential circuit 10 and the operating current Ils of the level shift circuit 20 are commonly controlled by the bias current source circuit 40. Accordingly, it is difficult to attain both a good distorting characteristic and a good through rate characteristic over a wide range of the supply voltage.
Specifically, in order to attain a good distortion characteristic, it is necessary to set the resistance values of the resistors 41 and 42 of the bias current source circuit 40 in consideration of the operating current Ils of the level shift circuit 20. In this case, however, the operating current Idiff cannot always have an appropriate magnitude over the wide range of the supply voltage, and hence, the through rate characteristic can be degraded. On the other hand, in order to attain a good through rate characteristic, it is necessary to set the resistance values of the resistors 41 and 42 of the bias current source circuit 40 in consideration of the operating current Idiff of the differential circuit 10. In this case, however, the operating current Ils of the level shift circuit 20 cannot always have an appropriate magnitude over the wide range of the supply voltage. Therefore, the distortion characteristic can be degraded, and in particular, when the supply voltage is high, the distortion characteristic can be more likely to be degraded.
Moreover, when the resistance values of the resistors 41 and 42 are set in the bias current source circuit 40 so that the voltage Vg can be relatively low, the voltage Vg is decreased to be lower than the threshold voltage Vt of the current source transistors 13 and 22 as the supply voltage is decreased. As a result, there is a possibility that the operational amplifier itself cannot be operated. Alternatively, in the case where the resistance values of the resistors 41 and 42 are set so that the voltage Vg can be relatively high, when the input signals SI1 and SI2 have low DC voltage levels, the differential current source transistor 13 is included in the unsaturation region because the source-drain voltage thereof becomes smaller than a value obtained by subtracting the threshold voltage from the gate voltage (Vds&lt;Vgs-Vt). As a result, the distortion characteristic can be degraded.